Problem: Which of these numbers is prime? ${21,\ 33,\ 63,\ 73,\ 96}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 21 are 1, 3, 7, and 21. The factors of 33 are 1, 3, 11, and 33. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 73 are 1 and 73. The factors of 96 are 1, 2, 3, 4, 6, 8, 12, 16, 24, 32, 48, and 96. Thus, 73 is a prime number.